use std::ffi::c_void;

mod sys {
    use super::*;

    pub type ScopedFunc = unsafe extern "C" fn(*mut c_void);

    pub unsafe fn scoped(callback: Option<ScopedFunc>, user_data: *mut c_void) {
        println!("Doing something fancy in C++");

        (callback.unwrap())(user_data);

        println!("Done executing callback.");
    }
}

struct Scope;

impl Scope {
    pub fn scoped<F>(mut func: F)
    where
        F: FnMut(&Scope),
    {
        unsafe {
            sys::scoped(
                Some(sure::<F>),
                &mut func as *mut _ as *mut std::ffi::c_void,
            );
        }
    }
}

unsafe extern "C" fn sure<T>(foo: *mut std::ffi::c_void)
where
    T: FnMut(&Scope),
{
    let foo = foo as *mut T;
    (*foo)(&Scope);
}

fn main() {
    Scope::scoped(|_| {
        println!("hello from callback");
    })
}